The use of subsea templates in offshore drilling and production operations is well known. Typically, templates are structurally sound frames incorporating spaced openings corresponding to the wellbores to be drilled. They also normally include means for supporting the well casing and other loads used in subsequent production operations. After a template is positioned on the sea floor over the formation of interest and secured in place by piles, a jacket for supporting the offshore platform is docked over the template and anchored. To carry out the jacket docking operation, docking piles, sometimes known as indexing piles, which previously have been anchored in the sea bed, are engaged by docking guides mounted on the jacket which slide down over the docking piles and guide the jacket into proper alignment.
Due to the force of waves and currents as well as wind loadings, it is very difficult to lower the jacket into place so that the docking guides are directly aligned with the docking piles. Normally, the jacket is misaligned to a degree which causes the docking guides to strike the docking piles before being moved into alignment. Because of the great mass of the jacket the impact can cause damage to the docking piles and the jacket. To prevent this from occurring sliding sleeves are commonly provided on the jacket to take up the impact and absorb the relative motion between the jacket and the docking pile. Typically, the motion absorbing assembly should allow up to a meter of motion with minimal loads into the docking guide. Although such an arrangement reduces the vertical loadings resulting from the impact, it adds considerably to the weight of the jacket. For economic reasons as well as for improving the maneuverability of the jacket during transportation to the site and subsequent lowering to the sea bed, it would be beneficial to reduce the jacket weight as much as practicable. This would have to be accomplished, however, without diminishing the ability to absorb relative vertical motion during a docking operation.
It is recognized that various types of damping or motion absorbing devices have been used in the past in connection with subsea installations, making use of various arrangements of resilient springs and elastomers. None of these designs, however, have shown themselves to be suitable for use in a jacket docking operation.